How do I use live motion, and paste a model onto it, like Gollum?
There are two approaches you could take -


Use motion capture. This involves using an actor that normally has markers attached to various parts of their body which can then be tracked from several camera angles and then used to control an armature that moves your 3D character.
Use motion tracking or match moving. This is often broken into two parts, camera tracking and object tracking. Camera tracking allows you to recreate the camera motion in 3D so that objects you add will follow the environment in front of the camera as if they was in front of the real camera during filming. Object tracking then follows an object that moves within the video, this could be a prop that an actor is holding that gets replaced with a high tech laser rifle. You can even track markers placed onto a persons face and use them to deform a 3D characters face. The points you track will be turned into empties within blender that you can use as reference to place objects or used to move objects within the scene so they match the video.


While the first option is getting cheaper it is still mostly left to movie studios with the budget, we can do the second approach within blender. This can be quite involved so you may want to watch some tutorials explaining it like track, match, blend or blenderguru's intro to camera tracking.

Some tips are to use some highly visible markers, in places that will get covered by your 3D objects. Place markers as far apart as possible. Account for rotations and the possibility of markers getting hidden and overlapping each other.